Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE TRANSPORT OF MOLECULES BETWEEN THE NUCLEUS AND THE CYTOSOL
657
nuclear pore
complex
lamins
DNA
nuclear envelope
FUSION OF ENVELOPED
CHROMOSOMES
inner nuclear membrane
outer nuclear membrane
PHOSPHORYLATION
OF LAMINS AND
NPC PROTEINS
phosphorylated
lamins
P
nuclear pore
complex
proteins
INTERPHASE NUCLEUS
P P
P P
P
P
P
P
P
P
P
P
P
LATE
TELOPHASE
daughter
chromosome
duplicated
chromosome
P
P
PROPHASE
nuclear envelope
fragment
FUSION OF NUCLEAR
ENVELOPE FRAGMENTS
EARLY TELOPHASE
DEPHOSPHORYLATION
OF LAMINS
to the chromosome surface, where they assemble into new NPCs. At the same
time, inner nuclear membrane proteins and dephosphorylated lamins bind again
to chromatin. ER membranes wrap around groups of chromosomes until they
form a sealed nuclear envelope (Movie 12.2). During this process, the NPCs start
actively re-importing proteins that contain nuclear localization signals. Because
the nuclear envelope is initially closely applied to the surface of the chromosomes,
the newly formed nucleus excludes all proteins except those initially bound to the
mitotic chromosomes and those that are selectively imported through NPCs. In
this way, all other large proteins, including ribosomes, are kept out of the newly
assembled nucleus.
MBoC6 m12.20/12.20
As we discuss in Chapter 17, the cloud of Ran-GTP surrounding chromatin is
also important in assembling the mitotic spindle in a dividing cell.
Summary
The nuclear envelope consists of an inner and an outer nuclear membrane that are
continuous with each other and with the ER membrane, and the space between the
inner and outer nuclear membrane is continuous with the ER lumen. RNA molecules,
which are made in the nucleus, and ribosomal subunits, which are assembled
there, are exported to the cytosol; in contrast, all the proteins that function in the
nucleus are synthesized in the cytosol and are then imported. The extensive traffic of
materials between the nucleus and cytosol occurs through nuclear pore complexes
(NPCs), which provide a direct passageway across the nuclear envelope. Small
molecules diffuse passively through the NPCs, but large macromolecules have to be
actively transported.
Proteins containing nuclear localization signals are actively transported into
the nucleus through NPCs, while proteins containing nuclear export signals are
transported out of the nucleus to the cytosol. Some proteins, including the nuclear
Figure 12–18 The breakdown and reformation
of the nuclear envelope and
lamina during mitosis. Phosphorylation
of the lamins triggers the disassembly
of the nuclear lamina, which initiates
the nuclear envelope to break up.
Dephosphorylation of the lamins reverses
the process. An analogous phosphorylation
and dephosphorylation cycle occurs
for some nucleoporins and proteins of
the inner nuclear membrane, and some
of these dephosphorylations are also
involved in the reassembly process. As
indicated, the nuclear envelope initially
re-forms around individual decondensing
daughter chromosomes. Eventually,
as decondensation progresses, these
structures fuse to form a single complete
nucleus.
Mitotic breakdown of the nuclear
envelope occurs in all metazoan cells.
However, in many other species, such as
yeasts, the nuclear envelope remains intact
during mitosis, and the nucleus divides by
fission.